1. Field of the Invention
The present invention relates to a cable, in particular to an electrical cable for power transmission or distribution at medium or high voltage.
More in particular, the present invention relates to an electrical cable which combines high impact resistance and compactness of its design.
In the present description, the term medium voltage is used to refer to a tension typically from about 10 to about 60 kV and the term high voltage refers to a tension above 60 kV (very high voltage is also sometimes used in the art to define voltages greater than about 150 or 220 kV, up to 500 kV or more); the term low voltage refers to a tension lower than 10 kV, typically greater than 100 V.
Furthermore, in the present description the term voltage class indicates a specific voltage value (e.g. 10 kV, 20 kV, 30 kV, etc.) included in a corresponding voltage range (e.g. low, medium or high voltage, or LV, MV, HV).
2. Description of the Related Art
Cables for power transmission or distribution at medium or high voltage generally have a metal conductor which is surrounded, respectively, with a first inner semiconductive layer, an insulating layer and an outer semiconductive layer. In the following of the present description, said predetermined sequence of elements will be indicated with the term of “core”.
In a position radially external to said core, the cable is provided with a metal shield (or screen), usually of aluminium, lead or copper, which is positioned radially external to said core, the metal shield generally consisting of a continuous tube or of a metallic tape shaped according to a tubular form and welded or sealed to ensure hermeticity.
Said metal shield has two main functions: on the one hand it provides hermeticity against the exterior of the cable by interposing a barrier to water penetration in the radial direction, and on the other hand it performs an electrical function by creating, inside the cable, as a result of direct contact between the metal shield and the outer semiconductive layer of said core, a uniform electrical field of the radial type, at the same time cancelling the external electrical field of said cable. A further function is that of withstanding short-circuit currents.
In a configuration of the unipolar type, said cable has, finally, a polymeric oversheath in a position radially external to the metal shield mentioned above.
Moreover, cables for power transmission or distribution are generally provided with one or more layers for protecting said cables from accidental impacts which may occur on their external surface.
Accidental impacts on a cable may occur, for example, during transport thereof or during the laying step of the cable in a trench dug into the soil. Said accidental impacts may cause a series of structural damages to the cable, including deformation of the insulating layer and detachment of the insulating layer from the semiconductive layers, damages which may cause variations in the electrical voltage stress of the insulating layer with a consequent decrease in the insulating capacity of said layer.
In the cables which are currently available in the market, for example in those for low or medium voltage power transmission or distribution, metal armours capable of withstanding said impacts are usually provided in order to protect said cables from possible damages caused by accidental impacts. Generally, said armours are in the form of tapes or wires (preferably made of steel), or alternatively in the form of metal sheaths (preferably made of lead or aluminum). An example of such a cable structure is described in U.S. Pat. No. 5,153,381.
European Patent No. 981,821 in the name of the Applicant, discloses a cable which is provided with a layer of expanded polymeric material in order to confer to said cable a high resistance to accidental impacts, said layer of expanded polymeric material being preferably applied radially external to the cable core. Said proposed technical solution avoids the use of traditional metal armours, thereby reducing the cable weight as well as making the production process thereof easier.
European Patent No. 981,821 does not disclose a specific cable core design. In practice, the constitutive elements of the cable core are chosen and dimensioned according to known Standards (e.g. to IEC Standard 60502-2 mentioned in the following of the present description).
According to the present invention, the Applicant observed that the use of an expanded protection of specific design can not only replace other types of protections, but also enable to use a smaller insulation size, thereby obtaining a more compact cable without reducing its reliability.
Moreover, cables for power transmission or distribution are generally provided with one or more layers which ensure a barrier effect to block water penetration towards the interior (i.e. the core) of the cable.
Ingress of water to the interior of a cable is particularly undesirable since, in the absence of suitable solutions designed to plug the water, once the latter has penetrated it is able to flow freely inside the cable. This is particularly harmful in terms of the integrity of the cable as problems of corrosion may develop within it as well as problems of accelerated ageing with deterioration of the electric features of the insulating layer (especially when the latter is made of cross-linked polyethylene).
For example, the phenomenon of “water treeing” is known which mainly consists in the formation of microscopic channels in a branch structure (“trees”) due to the combined action of the electrical field generated by the applied voltage, and of moisture that has penetrated inside said insulating layer. For example, the phenomenon of “water treeing” is described in EP-750,319 and in EP-814,485 in the name of the Applicant.
This means, therefore, that in case of water penetration to the interior of a cable, the latter will have to be replaced. Moreover, once water has reached joints, terminals or any other equipment electrically connected to one end of the cable, the water not only stops the latter from performing its function, but also damages said equipment, in most cases causing a damage that is irreversible and significant in economic terms.
Water penetration to the interior of a cable may occur through multiple causes, especially when said cable forms part of an underground installation. Such penetration can occur, for example, by simple diffusion of water through the polymeric oversheath of the cable or as a result of abrasion, accidental impact or the action of rodents, factors that can lead to an incision or even to rupture of the oversheath of the cable and, therefore, to the creation of a preferred route for ingress of water to the interior of the cable.
Numerous solutions are known for tackling said problems. For example, hydrophobic and water swellable compounds, in the form of powders or gel, which are placed inside the cable at various positions depending on the type of cable being considered, can be used.
For example, said compounds may be placed in a position radially internal to the metal shield, more precisely in a position between the cable core and its metal shield, or in a position radially external thereto, generally in a position directly beneath the polymeric oversheath, or in both the aforesaid positions simultaneously.
The water swellable compounds, as a result of contact with water, have the capacity to expand in volume and thus prevent longitudinal and radial propagation of the water by interposing a physical barrier to its free flow. Document WO 99/33070 in the name of the Applicant describes the use of a layer of expanded polymeric material arranged in direct contact with the core of a cable, in a position directly beneath the metallic screen of the cable, and possessing predefined semiconducting properties with the aim of guaranteeing the necessary electrical continuity between the conducting element and the metallic screen.
The technical problem faced in WO 99/33070 was that the covering layers of a cable are continuously subjected to mechanical expansions and contractions due to the numerous thermal cycles that the cable undergoes during its normal use. Said thermal cycles, caused by the diurnal variations in strength of the electric current being carried, which are associated with corresponding temperature variations inside the cable itself, lead to the development of radial stresses inside the cable which affect each of said layers and, therefore, also its metallic screen. This means, therefore, that the latter can undergo relevant mechanical deformations, with formation of empty spaces between the screen and the outer semiconducting layer and possible generation of non-uniformity in the electric field, or even resulting, with passage of time, in rupture of the screen itself. This problem was solved by inserting, under the metallic screen, a layer of expanded polymeric material capable of absorbing, elastically and uniformly along the cable, the aforementioned radial forces of expansion/contraction so as to prevent possible damage to the metallic screen. Furthermore, document WO 99/33070 discloses that, inside said expanded polymeric material, positioned beneath the metallic screen, a water swellable powder material is embedded, which is able to block moisture and/or small amounts of water that might penetrate to the interior of the cable even under said metallic screen.
As it will be recalled in more details in the following of the present description, in the same conditions of electrical voltage applied to a cable, cross-section thereof and insulating material of said cable insulating layer, a decrease of the cable insulating layer thickness causes the electrical voltage stress (electrical gradient) across said insulating layer to increase. Therefore, generally the insulating layer of a given cable is designed, i.e. is dimensioned, so as to withstand the electrical stress conditions prescribed for the category of use of said given cable.
Generally, even though a cable is designed to provide for a thickness of the insulating layer which is larger than needed so that a suitable safety factor is included, an accidental impact occuring on the external surface of the cable can cause a permanent deformation of the insulating layer and reduce, even remarkably, the thickness thereof in correspondence of the impact area, thereby possibly causing an electrical breakdown therein when the cable is energized.
In fact, generally the materials which are typically used for the cable insulating layer and oversheath elastically recover only part of their original size and shape after the impact. Therefore, after the impact, even if the latter has taken place before the cable is energized, the insulating layer thickness withstanding the electric stress is inevitably reduced.
Furthermore, when a metal shield is present in a position radially external to the cable insulating layer, the material of said shield is permanently deformed by the impact, fact which further limits the elastic recover of the deformation so that the insulating layer is restrained from elastically recovering its original shape and size.
Consequently, the deformation caused by an accidental impact, or at least a significant part thereof, is maintained after the impact, even if the cause of the impact itself has been removed, said deformation resulting in the decrease of the insulating layer thickness which changes from its original value to a reduced one. Therefore, when the cable is energized, the real insulating layer thickness which bears the electrical voltage stress (Γ) in the impact area is said reduced value and not the starting one.